feat(mbyte): support extended grapheme clusters including more emoji

Use the grapheme break algorithm from utf8proc to support grapheme
clusters from recent unicode versions.

Handle variant selector VS16 turning some codepoints into double-width
emoji. This means we need to use ptr2cells rather than char2cells when
possible.

test/old/testdir/test_functions.vim

@@ -3663,7 +3663,7 @@ func Test_string_reverse()
     call assert_equal('', reverse(v:_null_string))
     for [s1, s2] in [['', ''], ['a', 'a'], ['ab', 'ba'], ['abc', 'cba'],
                    \ ['abcd', 'dcba'], ['«-«-»-»', '»-»-«-«'],
-                   \ ['🇦', '🇦'], ['🇦🇧', '🇧🇦'], ['🇦🇧🇨', '🇨🇧🇦'],
+                   \ ['🇦', '🇦'], ['🇦🇧', '🇦🇧'], ['🇦🇧🇨', '🇨🇦🇧'],
                    \ ['🇦«🇧-🇨»🇩', '🇩»🇨-🇧«🇦']]
       call assert_equal(s2, reverse(s1))
     endfor